1. Field of the Invention
The present invention relates to a method and apparatus for slow reverse playback, and more particularly, to a method and apparatus for normal reverse playback.
2. Description of the Related Art
Accompanying the introduction of High Definition (HD) broadcasting, digital television (TV) sets have rapidly become popular. More recently, additional functions have been introduced in digital TVs, and premium-class digital TV sets and Audio/Video (AV) devices with high image quality have been introduced in the market. Moreover, during the same time, demand for a Personal Video Recording (PVR) function has increased, and TV sets employing the PVR function are available. The PVR function has a trick mode and a time shift function.
The PVR trick mode includes a slow reverse playback mode. A trick mode decoding apparatus for the slow reverse playback mode is disclosed in Korean Patent Publication No. 10-0335060. FIG. 1 is a block diagram of the trick mode decoding apparatus disclosed in the related art. Referring to FIG. 1, the trick mode decoding apparatus includes a Digital Storage Medium (DSM) 101, such as a Compact Disc Read Only Memory (CD-ROM), a Digital Audio Tape (DAT), or a Digital Versatile Disc (DVD), a video decoder 102 for Moving Picture Experts Group (MPEG) decoding, and a memory 103 storing image data for display. The DSM 101 repeatedly transmits an MPEG bitstream corresponding to a single Group Of Pictures (GOP) to the video decoder 102. The memory 103 has the capacity to store pictures corresponding to three frames.
FIG. 2 is a conceptual diagram for describing the slow reverse playback mode of the apparatus illustrated in FIG. 1. If slow reverse playback starts in the apparatus illustrated in FIG. 1, the DSM 101 repeatedly provides the same GOP to the video decoder 102 a number of times, corresponding to the number of pictures that must be displayed. For instance, if the slow reverse playback mode begins when an Mth picture of a GOP(0) is displayed, the DSM 101 repeatedly transmits a bitstream of the GOP(0) to the video decoder 102 (M−1) times. More particularly, when the bitstream of the GOP(0) is transmitted to the video decoder 102 for the first time after the slow reverse playback mode begins, the video decoder 102 can decode first to (M−1)th pictures of the GOP(0). Next, when the bitstream of the GOP(0) is transmitted to the video decoder 102 for the second time, the video decoder 102 can decode first to (M−2)th pictures of the GOP(0). Here, M and (M−1) denote the order of pictures of the GOP(0) in a forward play mode. In this manner, the video decoder 102 decodes the bitstream of the GOP(0) (M−1) times and decodes a bitstream of a GOP(−1) from an Nth picture. According to the MPEG standard, an I picture denotes an intra coded picture, a P picture denotes a predictive coded picture, and a B picture denotes a bidirectionally-predictive coded picture. In this case, the video decoder 102 decodes only reference pictures including I and P pictures and a picture to be displayed next. A decoding operation of the video decoder 102 will now be described in more detail with reference to FIG. 3.
FIG. 3 illustrates a stream order and a display order of pictures of a single GOP. In FIG. 3, the display order indicates the order in which I, P, and B pictures are displayed on a screen, and the stream order indicates the order in which I, P, and B pictures are included in a bitstream. B0+, B1+, and I2+ pictures having a superscript “+” indicate pictures of a GOP(X+1). It is assumed in FIG. 3 that each GOP is comprised of 9 pictures. In each GOP, a number next to each capital I, P, or B indicates the display order of the corresponding pictures of the GOP. Thus, pictures of a GOP(X) are displayed in the order of B0, B1, I2, B3, B4, P5, B6, B7, and P8 in the case of forward playback. In the bitstream, the pictures of the GOP(X) are included in the order of I2, B0, B1, P5, B3, B4, P8, B6, and B7.
In the slow reverse playback mode, the pictures of the GOP(X) illustrated in FIG. 3 must be displayed in the order of P8, B7, B6, P5, B4, B3, 12, B1, and B0. Meanwhile, according to the MPEG standard, a P picture needs an I or P picture as a reference picture, and a B picture needs an I picture and/or a P picture as reference pictures. Thus, in order to decode a P or B picture located at the end of a stream order among the pictures in a GOP, reference picture(s) for the P or B picture must be decoded first. As described above, the memory 103 illustrated in FIG. 1 has the capacity to store 3 pictures. Thus, if the Mth picture being displayed when switching to reverse playback is the picture B7 of the GOP(X) illustrated in FIG. 3, the video decoder 102 obtains only the pictures I2, P5, P8, and B6 by decoding the bitstream received from the DSM 101. That is, the video decoder 102 skips the pictures B0, B1, B3, and B4 of the received encoded pictures without decoding.
If a picture B7 is currently displayed, after the memory 103 stores data of the pictures I2, P5, and P8, the data of the picture I2 is deleted from the memory 103 in order to prepare the space for storing data of the picture B6. The picture B6 is decoded using the data of the decoded pictures P5 and P8 stored in the memory 103, and the decoded picture B6 is stored in the memory 103. The decoded picture B6 stored in the memory 103 is transmitted for display. While the picture B6 is repeatedly displayed, the video decoder 102 decodes the bitstream of the GOP(X) received from the DSM 101 and stores the decoded pictures I2 and P5 in the memory 103. The picture P5 stored in the memory 103 is displayed. While the picture P5 is displayed, the video decoder 102 obtains the decoded pictures I2 and P5 by decoding the bitstream of the GOP(X) received from the DSM 101 and obtains the decoded picture B4 using the decoded pictures I2 and P5. While the picture B4 is displayed, the DSM 101 transmits the bitstream of the GOP(X) to the video decoder 102, and the video decoder 102 decodes the bitstream of the GOP(X) in order to obtain the reference pictures I2 and P5 and the picture B3. In this manner, the pictures are displayed on a display block (not shown) in the order of B7, B6, P5, B4, B3, I2, B1, and B0.
However, when the apparatus illustrated in FIG. 1 performs slow reverse playback, it must repeatedly decode a bitstream until a picture is displayed. For example, in order to display the picture B7 or B6, the three reference pictures I2, P5, and P8 and the subject picture B7 or B6 must be decoded. In another case, in order to display the picture B3 or B4, the two reference pictures I2 and P5 and the subject picture B3 or B4 must be decoded. Thus, as the speed of slow reverse playback approaches normal speed, i.e., 1× speed, the time required to decode the bitstream as set forth above becomes insufficient. As a result, the apparatus illustrated in FIG. 1 cannot implement normal (1×) reverse playback.
Furthermore, the apparatus illustrated in FIG. 1 repeatedly decodes reference pictures (I and P pictures) from a first picture of each GOP by using the memory 103, which can store three image frames. Thus, as more reference pictures are included in a GOP, more time is needed for decoding pictures to be displayed. Consequently, the decoding performance of an MPEG decoder must be excellent when the number of pictures in a GOP increases.
In order to solve this problem, a memory buffer used to store all the pictures included in a GOP may be implemented in a video decoder. However, this configuration would use memory inefficiently, since only reference pictures I and P are needed to decode each B picture.